This invention concerns a method of producing a ultra low carbon steel at high nitrogen concentration, particularly, a ultra low carbon steel at high concentration of solid-solute N. The ultra low carbon steel at high nitrogen concentration can be applied, for example, with rolling to obtain a ultra low carbon steel sheets (thin steel sheets) of high age hardening property. The high nitrogen ultra low carbon steel sheets can be used for portions such as of automobile structural parts, which require structural strength, particularly, strength and/or rigidity upon deformation.
As one of steel sheets suitable, for example, to automobile structural parts, steel sheets which have favorable workability and can be improved for the strength by an aging heat treatment after once being formed (hereinafter referred to as age hardening property) have been proposed. In the steel sheets, the strength can be improved by applying forming such as press forming in a relatively soft state before the age hardening treatment into a desired shape and then applying an aging heat treatment such as baking. As the steel for use in sheets of this type, a ultra low carbon steel at Cxe2x89xa60.0050 mass % is considered suitable with a view point of the workability, and it has been proposed a composition in which solid solute N can be present, for example, by 0.0030 mass % or more, preferably, 0.0050 mass % or more in steel sheets with a view point of aging property.
However, for refining to form a steel of such excellent workability, Al is generally added in view of deoxidation (such a steel is referred to as aluminum killed steel). Further, for refining of crystal grain size in the ultra low carbon steel, a technique, for example, of adding Nb or B into the steel has often been adopted. Since the elements described above form nitrides, it is necessary for insuring solid solute N in the steel sheets, to control the nitrogen concentration for compensation of the nitrogen content that is converted into nitrides upon steel making. For example, in a case where Al concentration in the steel is 0.015 mass % or more, it is necessary for high N concentration of about 0.0120 mass % or more in order to ensure a sufficient content of solid solute N.
As a method of producing a steel of high N concentration, Japanese Patent Laid-Open No. 91317/1986 discloses a method of blowing a nitrogen gas from a submerged lance into a molten steel in a ladle refining furnace under an oxygen free atmosphere. However, since this method is a treatment in the ladle refining furnace, it is difficult to apply, for example, a vacuum degassing treatment, so that it is extremely difficulty to obtain a ultra low carbon steel.
On the other hand, for the method of producing a high N steel of conducting the vacuum degassing treatment, Japanese Patent Publication No. 34848/1980. Japanese Patent Laid-Open No. 25919/1981 and Japanese Patent Laid-Open No. 28319/1989 disclose methods of controlling the pressure in a vacuum vessel to a pressure equilibrated with an aimed N concentration after the vacuum degassing step, utilizing a nitrogen gas as a part or entire of a gas to be blown into a molten steel, and keeping for a predetermined period of time, thereby adding nitrogen sufficiently.
However, the nitrogen injection method by a nitrogen gas involves a drawback that the nitrogen increasing rate is slow. Particularly, in the steel material used for steel sheets to be worked, since Cr concentration is low different from stainless steel and the like, the nitrogen solubility is low and it is difficult to attain a processing speed suitable to industrial production. While the disclosed technique propose an attempt of increasing nitrogen up to an equilibrated nitrogen concentration by increasing the pressure in the vacuum vessel, this also requires a long time to reach the equilibrated nitrogen concentration when the initial nitrogen concentration is low.
For example, in a case of a pressure at 1xc3x97104 Pa in a vacuum vessel where the equilibrated nitrogen concentration is 0.0150 mass %, increase is only up to about 0.0100 mass % by a treatment for 15 min when the initial nitrogen concentration is about 0.0080 mass %. Accordingly, when the aimed nitrogen concentration is, for example, 0.0120 mass % or more as described above, it is extremely difficult to attain the aimed value by the injection of the nitrogen gas. Although the nitrogen concentration may be increased by increasing the pressure in the vacuum vessel, the pressure in the vacuum vessel as exceeding 2.0xc3x97104 Pa lowers a stirring force for the molten steel in a vacuum vessel or a ladle to hinder the homogeneity in the molten steel.
A method of blowing a nitrogen gas or a nitrogen-Ar gas mixture in a vacuum degassing apparatus under a reduced pressure to control the pressure in the vacuum vessel thereby controlling the nitrogen concentration in the molten steel has been disclosed in Japanese Patent Laid-Open No. 17321/2000, Japanese Patent Laid-Open No. 17322/2000, Japanese Patent Laid-Open No. 34513/2000 and Japanese Patent Laid-Open No. 100211/1996. However, like the techniques described previously, nitrogen increasing rate in the injection of nitrogen by the nitrogen gas is slow and it takes a long processing time in ordinary steels, which is not practical.
Further, Japanese Patent No. 2896302 discloses a technique of changing the pressure in a vacuum vessel and decreasing nitrogen to less than an aimed nitrogen concentration of a molten steel and then adding a nitrogen-containing alloy to conduct fine control as far as the aimed nitrogen concentration. To ensure the aimed nitrogen concentration by the addition of the nitrogen-containing alloy brings about the change of the steel composition by the alloy. For example, it results in a problem that the C concentration in the molten steel is increased by C contained in the alloy. At the same time, the nitrogen-containing alloy with controlled composition is expensive and it is difficult aside from special steels, to adopt such an uneconomical method for steel species as in steel sheets put to ordinary working that require mass production and production at reduced cost.
Further, Japanese Patent Laid-Open No. 216439/1995 discloses a method of blowing a nitrogen gas into a molten steel in primary decarburization refining and secondary vacuum decarburization refining thereby refining to form a steel at a high nitrogen content of 0.0100 mass % or more in a ultra low carbon steel at 0.0050 mass % or less. However, when the denitridation reaction along with the decarburizing treatment in the secondary refining is taken into consideration, this method requires addition of a great amount of nitrogen in total compared with a case of adding nitrogen only in the secondary refining. Accordingly, in conjunction with the low processing rate for the high nitrogen treatment, only low production efficiency can be expected by the gas in this method.
Further, it has been actually difficult to attain an N content of 0.0120 mass % or more in a ultra low carbon steel at: Cxe2x89xa60.005 mass % by any of the methods described above.
This invention proposes a method of producing, at a reduced cost and with high productivity, a steel for obtaining a steel sheet to be worked which contains nitrogen at high concentration (solid-solute nitrogen) and ultra low carbon content. The steel obtained by the method according to this invention is served particularly for application use in which an aging heat treatment is applied for improving the strength after forming such as press forming and which is suitable as a rolling material for steel sheets having excellent age hardening property.
The present inventors have made earnest studies for attaining the foregoing object and, as a result, have found a new subject, in producing a high nitrogen steel in a ultra low carbon aluminum killed steel, that AlN is precipitated to cause AlN-induced surface crackings in cast slabs or sheet bars during continuous casting and hot rolling unless the amount of Al added to the steel upon deoxidation is controlled appropriately. Then, it has been succeeded in solving the problems described above, by providing an upper limit for the concentration of Al and N to prevent lowering of the product yield and ensure the productivity.
Further, the present inventors have succeeded in obtaining a desired high nitrogen content efficiently while ensuring the reduced cost and productivity, particularly, the production speed, by the procedures of optimizing the concentration of nitrogen and carbon after primary refining, controlling denitridation along with decarburization in secondary refining in a vacuum degassing facility and, optionally adding nitrogen. It is preferred in view of the cost and the productivity, to control the amount of nitrogen in the primary refining by the blowing of a nitrogen-containing gas or addition of a nitrogen-containing alloy, to control denitridation in the secondary refining by blowing of a suitable nitrogen-containing gas or control the amount of oxygen in the steel and to adjust nitrogen upon subsequent Al killed treatment by the nitrogen-containing gas and an composition-controlled nitrogen containing alloy.
That is, this invention provides a method of producing a rolling material for use in ultra low carbon steel sheets of high age hardening property in producing a rolling material for use in ultra low carbon steel sheets at: Cxe2x89xa60.0050 mass %, characterized by applying primary decarburization refining to molten iron from a blast furnace, controlling the composition in the molten steel after the primary decarburization refining to a range satisfying the following relation (1), then conducting secondary decarburization refining to a ultra low carbon concentration region at: Cxe2x89xa60.0050 mass % so as to satisfy the following relation (2) in a vacuum degassing facility, subsequently conducting deoxidation by Al so as provide: Alxe2x89xa70.005 mass % after deoxidation, further, controlling the composition such that N: 0.0050-0.0250 mass % and the N concentration satisfies the following relation (3) and, successively, casting the thus composition-controlled molten steel at continuous casting process.
Note:
[mass % N]xe2x88x920.15[mass % C]xe2x89xa70.0060xe2x80x83xe2x80x83(1) 
xe2x80x83xcex94N/xcex94Cxe2x89xa60.15xe2x80x83xe2x80x83(2)
in which
xcex94N: reduction amount of the N concentration in steel in the secondary decarburization refining (mass %)
xcex94C: reduction amount of the C concentration in steel in the secondary decarburization refining (mass %)
[mass % Al]xc2x7[mass % N]xe2x89xa60.0004xe2x80x83xe2x80x83(3) 
For improving the age hardening property of steel sheets obtained from the steel according to this invention, it is preferred that the N concentration further satisfies, in the composition control, the following relation (4):
[mass % N]xe2x89xa70.0030+14/27[mass % Al]+14/93[mass % Nb]+14/11[mass % B]+14/48[mass % Ti]xe2x80x83xe2x80x83(4), 
thereby ensuring an appropriate amount of solid solute N. The steel according to this invention does not necessarily contain Nb, B and Ti and the value for the concentration of the not contained element in the formula described above is calculated as zero.
This invention is not restricted to the steels satisfying the relation (4) but is suitable to the production, particularly, of high nitrogen steels at N: 0.0120 mass % or more.
During the secondary decarburization refining, it is preferred to blow a gas that contains a nitrogen gas, for example, a nitrogen gas or a gas mixture of nitrogen and argon at a nitrogen gas flow rate: 2 Nl/minxc2x7t or more into the molten steel to provide: xcex94N/xcex94Cxe2x89xa60.15. Further, it is preferred to control the N concentration also in deoxidation by Al in a vacuum degassing facility after the secondary decarburization refining by blowing a gas that contains a nitrogen gas at a nitrogen gas flow rate: 2 Nl/minxc2x7t or more. There is no particular restriction on the method of blowing the gas into the molten steel, and may be a method of blowing from a ladle not only from a snorkel or may be a method of blowing the gas to the surface of the molten steel.
Further, the gas that contains the nitrogen gas further contains preferably a reducing gas, for example, a hydrogen gas with a view point of the efficiency for nitrogen supply. The reducing gas is preferably 5 to 50 vol % (normal temperature normal pressure) of the gas that contains the nitrogen gas.
The nitrogen containing gas that contains the reducing gas can be used also for increasing the nitrogen concentration during primary refining.
Further, it is also preferred to control the concentration of oxygen in the molten steel to 0.0300 mass % or more during secondary decarburization refining to provide: xcex94N/xcex94Cxe2x89xa60.15.
Further, the composition of the molten steel before the secondary decarburization refining preferably satisfies the following relation (5):
[mass % N]xe2x88x920.15[mass % C]xe2x89xa70.0100xe2x80x83xe2x80x83(5) 
As a specific numerical value, the composition in the molten steel before the secondary decarburization refining is preferably Nxe2x89xa70.0080 mass %. More preferably, it is controlled to as: Nxe2x89xa70.0100 mass %.
In the control for the ingredients in the molten steel before the secondary decarburization refining, it is preferred to control the N concentration by adding an N-containing alloy to the molten steel after the primary decarburization refining and before the secondary decarburization refining.
Further, it is preferred to suppress lowering of the N concentration by adjusting the pressure in the vacuum vessel to 2xc3x97103 Pa or more during deoxidation by Al (killed treatment) in the vacuum degassing facility after the secondary decarburization refining.
Further, it is preferred to control the N concentration by adding an N-containing alloy at: [mass % C]/[mass % N]xe2x89xa60.1 into the molten steel during deoxidation by Al in the vacuum degassing facility after the secondary decarburization refining. This is preferably conducted with an aim of fine control for the N concentration.
The composition of the molten steel controlled with the composition preferably contains Si: 1.0 mass % or less, Mn: 2.0 mass % or less and total oxygen: 0.0070 mass % or less and contains one or more of Nb: 0.0050 to 0.0500 mass %, B: 0.0005 to 0.0050 mass % and Ti: 0.070 mass % or less (including zero), with the substantial balance being Fe.